The prevalence of childhood and adolescent obesity has reached an all-time high of 17%. There is a pressing need to better understand the behavioral mechanisms that control appetite and eating in children and to identify children who are most susceptible to overeating in the current obesogenic food environment. Obesogenic environments offer convenient access to large portions of palatable, energy-dense foods. The portion size of foods has been identified as an important determinant of energy intake in adults and children. While portion size effects have been found in heterogenous groups of healthy individuals, some evidence points to possible individual differences in the magnitude of children's responsiveness to large food portions. For example, data from a previous laboratory study suggest that overweight and obese children may be particularly vulnerable to overeating when presented with large food portions. Further, the magnitude of a portion size effect on intake may also depend on how reinforcing (or rewarding) children find food to be. It is possible that the relative reinforcing value of food may interact with environmental cues, such as the portion size of food, to affect children's energy intake when large portions of palatable, energy-dense foods are available. The primary aim of this study is to compare the effects of increasing the portion size of all foods and beverages served at a meal on energy intake in normal-weight and obese children. A second aim is to test if children who find food very reinforcing relative to an appealing nonfood alternative will show a significantly greater increase in energy intake in response to increasing portion sizes than will children who find food less reinforcing. The proposed study will also examine if maternal child feeding practices, such as using food as a reward, is associated with the relative reinforcing value of food in children. Previous studies have identified important determinants of energy intake in children. Under free-living conditions, environmental factors interact with individual predispositions to excessive weight gain to affect eating behavior. By examining environmental influences and individual-level susceptibility to overeating together, we may be able to identify risk factors for overeating that may be obscured when either one of these determinants is examined in isolation. The proposed study will extend previous findings to test how environmental factors, such as food portion size, may interact with individual differences in weight status or food reinforcement to affect energy intake in children. These data will translate to the development of dietary and environmental strategies for the prevention and treatment of childhood obesity. PUBLIC HEALTH RELEVANCE: In order to develop effective strategies to prevent and treat childhood obesity, there is a pressing need to identify modifiable eating traits and environmental factors which promote overeating in children. The proposed study will elucidate how children's weight status and relative reinforcing value of food may interact with aspects of the food environment (portion size) to affect energy intake.